The present invention relates to an improved suction detector which can accurately detect suction and removal of, for example, semiconductor parts or electronic parts, and a suction/transfer apparatus which incorporates the same.
Conventionally, a suction detector utilizes a pressure difference in a fluid and detects suction/removal of, for example, semiconductor parts or electronic parts during conveyance of the parts. A suction/transfer apparatus incorporating such a detector includes a suction nozzle, with the suction detector itself being located between the suction nozzle and a vacuum device. When an object is chucked by a reduced pressure, the suction detector detects a reduction in the pressure and hence suction of the object. When the object is removed by increasing the pressure, the suction detector detects an increase in the pressure and hence removal of the object. Examples of a pressure sensor used in the above suction/transfer apparatus are a mechanical type which varies a contact position in accordance with a change in pressure, and an electric type which uses a semiconductor transducer. FIG. 1 shows an arrangement of a conventional suction detector which uses a semiconductor transducer.
Referring to FIG. 1, semiconductor transducer 111 for converting a pressure value of a fluid into a resistance is connected between terminals 112 and 113 through diode 114 and converter 115 for converting a constant voltage into a constant current. An output terminal of transducer 111 is connected to the inverting input terminal of operational amplifier 117 as a comparator through operational amplifier 116 as an amplifier. The non-inverting input terminal of amplifier 117 is connected to a variable terminal of variable resistor 118 provided between converter 115 and terminal 113. The output terminal of amplifier 117 is connected to the base of transistor 119, the emitter of transistor 119 is connected to terminal 113, and its collector is connected to terminal 120. Diode 121 is connected between the emitter and the collector of transistor 119 so that an anode of diode 121 is connected to the emitter of transistor 119. Light-emitting diode 122 for display is connected between the base of transistor 119 and terminal 113.
An operation of the circuit shown in FIG. 1 will be described below, with reference to FIG. 2.
First, by adjusting resistor 118, level A is set at a constant value not crossing a drift value of a pressure. However, a resistance of transducer 111 varies in accordance with the pressure. Therefore, level B of the inverting input terminal of amplifier 117 varies in accordance with a pressure value of a fluid. Since the pressure of the fluid is high when an object is not chucked to the nozzle, an output voltage is low and level B of the inverting input terminal of operational amplifier 117 becomes lower than level A. However, since the fluid pressure is low when an object is chucked by a suction nozzle, an output voltage of transducer 111 is high, and hence level B of the inverting input terminal of the amplifier 117 becomes higher than level A. Therefore, when the object is not chucked, an output from the amplifier 117 becomes negative, and no current is supplied to the base of transistor 119. However, when the object is chucked, an output from amplifier 117 becomes positive, and a current is supplied to the base of transistor 119. Therefore, transistor 119 is turned on, and diode 122 emits light to indicate that the object is chucked. Thus, suction is repeatedly detected in accordance with a suction operation of the object.
However, in the above circuit, pressure level B is varied due to the presence of a vacuum device while level A is fixed as shown in FIG. 2. Therefore, when level B is at P2 or P4, suction can be detected, but when level B is at P1, P3, or P5, suction cannot be detected because level A is higher than level B. As a result, although an object is chucked, no suction is detected and hence the object cannot be transferred.
In addition, if an orifice of the suction nozzle is narrow, a pressure difference in suction becomes small. Therefore, since level A become higher than level B, suction cannot be detected. Moreover, since a size of the orifice is varied by dirt of the suction nozzle and the like during an operation, detection accuracy is degraded or suction cannot be detected.
Since level A is fixed, detection time varies upon pressure change in pipe 4, and in a high-speed machine in particular, the detection timing error may occur.